Recently, there has been a growing concern regarding problems related to air pollution. One major source of such pollution are emissions from power generating plants. For example, oxides of nitrogen and sulfur are produced in power station boilers by the combustion of the fuel used in the boilers. The nitrogen oxides may be produced by pyrolysis of nitrogen containing compounds in the fuel and may also be produced by reactions of N.sub.2 and O.sub.2 at elevated temperatures (called nitrogen fixation). Normally the nitrogen oxides are present as nitric oxide (NO), but also other nitrogen oxides, especially NO.sub.2, are usually present in small quantities. The oxides of nitrogen are referred to herein as NO.sub.x. The oxides of sulfur are mainly present as SO.sub.2 with minor amounts of SO.sub.3 present. The oxides of sulfur are referred to herein as SO.sub.x.
The SO.sub.x and NO.sub.x emissions are desirably removed from the flue gas prior to discharge into the atmosphere because SO.sub.x combines with atmospheric water vapor to form acids of sulfur. In a like manner, NO.sub.x combines with atmospheric water vapor to form acids of nitrogen. These acids then fall to the earth as "acid rain", undesirably making the environment more acidic. The nitrogen oxides also contribute to air pollution by taking part in the formation of photochemical smog.
One method of providing relatively low levels of SO.sub.x and NO.sub.x emission is to use clean fuels, such as light fuel oil or natural gas, which are expensive. Less costly fuels, such as coal, produce much higher levels of uncontrolled NO.sub.x and SO.sub.x pollution. If a low-cost method of achieving simultaneous NO.sub.x /SO.sub.x control were available, then dirty fuels, such as coal, could be used with corresponding economic benefit to the users.